One such assembly using a first, hollow torsion bar and a second torsion bar arranged in the interior of the first torsion bar is known from German published patent application 196 53 510. This unit allows, for instance in a collision, that belt webbing can be withdrawn in overcoming a predetermined resisting force to diminish peak loads in the seat belt and thus the resulting risk of injury to the restrained vehicle occupant. The resistance to withdrawal of the belt webbing is determined by the moment of torsional resistance effective in each case. This resistance may be adapted to the individual requirements by alternatively activating one or the other torsion bar or by commonly activating both thereof.
The disadvantage with this known belt retractor is that varying the resulting characteristic of the resisting force over the length of the withdrawn belt webbing necessitates a complicated and bulky mechanism which switches the various torsion bars as desired with the necessary safety.
The object of the invention consists in providing a unit for limiting the force in a belt retractor in which the resulting characteristic of the belt webbing withdrawal force can be varied over the length of the withdrawn belt webbing as desired, without making any complicated external control mechanisms necessary for the switching action.